During antigen-driven differentiation of immunoglobulin-producing cells, a switch from mu to gamma, delta, or a heavy chain synthesis can occur. This protein switch is the result of a DNA deletion that moves a V region from a donor C-mu to a C-gamma, C-epsilon, or C-alpha gene. This deletion begins and ends in switch segments, DNA regions composed of tandemly repeated sequences. Our investigation aims to study in detail the sequence of murine S-gamma segments. There is interest in the organization of this switch segment, particularly in the presence of unique sequences among the tandem repeats. Our plans include: (1) examining carefully the homology relationships among S-gamma-3, S-mu, and S-gamma-1 so that we can evaluate the role of S-gamma-3 in mu to gamma switches; (2) proposing to clone and sequence the switch recombination sites in five expressed gamma-3 genes and five expressed gamma-1 genes (from both plasmacytomas and hybridomas); (3) asking where in the switch segments (in unique or repeated sequences) the switch recombinations take place and if there is a recognition sequence for gamma-3 switch sites different from a recognition sequence for gamma-1 switch sites. If the heavy chain switch is s sister chromatid exchange, sequences between the donor and recipient switch segments may not be deleted; (4) attempting to identify, clone, and sequence DNA fragments that are derived from between donor and recipient switch segments but are retained in the DNA of immunoglobulin-producing hybridomas; (5) understsnding the specificity and genetics of switch recombinases, we intend to select a cell line that constitutively produces these enzymes. Our research suggests that switch recombinases catalyze c-myc-switch segment rearrangements. If translocations to the light chain locus in plasmacytomas reflect c-myc-light chain rearrangements, it would be difficult to understand the role of switch recombinases in this rearrangement; and (6) screening plasmacytoma DNAs for c-myc-kappa light chain recombinations and determining the nature of such recombinations if they exist. (IS)